This volume button thing just happened to me. I saw the mentions of dents, but my iPad hasn't fallen, and it's in a Belkin case. I took the case off, and sure enough, the metal around the switch appeared to be slightly bent up toward the screen, pushing the screen just barely noticeably up. Using the side of my knuckle, I firmly massaged the screen over the switch downward, and managed to put it back in order. The little strip of metal is very flimsy. It was a simple fix for me.
Thank you so much! You are a genius. Such a simple fix but I would never have thought of it. I couldn't understand why it had gone wrong as I hadn't dropped or dented it in any way. I can now play videos again. Woo Hoo!